class Solution21_2 {
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
         if(list1 == null ) return list2 ;
         if(list2 == null ) return list1 ; 
          ListNode newHead = null ;
          if(list1.val < list2.val){
           newHead = list1 ;
           newHead.next = mergeTwoLists(list1.next , list2) ; 
          }else{
           newHead = list2 ;
           newHead.next = mergeTwoLists(list1 , list2.next); 
          }
          return newHead ;
    }
}